Polishing tools are constructed by attaching a polishing pad which directly contacts the object of polishing (e.g., a semiconductor wafer) to a polishing head which constitutes a polishing pad holding member. Polishing pads are known in which the polishing pad is attached to the pad attachment surface of the polishing head by means of an adhesive agent, adhesive tape or the like, while the polishing pad is attached to a plate-form member that can be attached to or detached from the polishing head by vacuum suction, so that the entire plate-form member can be replaced when the polishing pad is replaced. In the case of polishing tools that have such a construction, replacement of the polishing pad can be accomplished by means of attachment and detachment utilizing vacuum suction of the plate-form member on the polishing head instead of bothersome removal and attachment of the polishing pad; accordingly, the efficiency of polishing work can be improved. Furthermore, such a construction in which the object is held by vacuum suction has also been applied to a mechanism which holds the object of polishing on the chuck (for example, see Japanese Patent Application Laid-Open No. 11-48138).
In the case of a polishing tool of the type in which a plate-form member to which the polishing pad is attached as described above (the assembly in which the polishing pad is attached to this plate-form member will hereafter be referred to as the polishing body) is attached and detached by vacuum suction to and from the polishing body attachment surface (hereafter referred to as the attachment surface) of the polishing head constituting a holding member, it is necessary to detect whether or not a polishing body is held on the polishing head by suction prior to the initiation of the polishing of the object of polishing. Such detection of the suction of the polishing body has conventionally been accomplished by installing a pressure sensor in the vacuum duct connecting the vacuum source and the polishing head, and detecting suction on the basis of the pressure inside the vacuum duct measured by this pressure sensor. For example, in a case where it is known that the pressure inside the vacuum duct is approximately −80 kPa in a state in which no polishing body is attached to the polishing head by suction, and that the pressure inside the vacuum duct is approximately −30 kPa in a state in which is polishing body is attached to the polishing head by suction, a pressure of −60 kPa is set as a threshold value, and it is judged that no polishing body is attached to the polishing head by suction in cases where the pressure inside the vacuum duct measured by the pressure sensor exceeds this threshold value (i.e., in cases where the degree of vacuum is lower than the threshold value), while it is judged that a polishing body is attached to the polishing head by suction in cases where the pressure inside the vacuum duct measured by the pressure sensor is less than the threshold value (i.e., in cases where the degree of vacuum is higher than the threshold value).
However, in the case of a method in which it is detected whether or not a polishing body is held on the polishing head by suction on the basis of the pressure inside the vacuum duct connecting the vacuum source and polishing head measured by a pressure sensor installed in this vacuum duct as described above, if the vacuum duct is branched and these branched portions of the vacuum duct are connected to other vacuum suction devices, the vacuum suction force of the vacuum source that is distributed to the polishing tool is weakened; as a result, there may be cases in which the pressure inside the vacuum duct that is measured in a state in which a polishing body is attached by vacuum suction drops and approaches the threshold value, so that it becomes difficult to discriminate a state in which a polishing body is held on the polishing head by suction.